Add environment variable for using a custom CA (for the additional-certificates configMap to work)

[abctaylor]

Description of the change

The current commented-out additionalVolumes example that loads in a configMap for supplying a custom enterprise root CA doesn't work on its own - we need to tell node to use it via a custom environment variable.

Benefits

Using a custom CA works fully with this.

Possible drawbacks

N/A - we already have commented-out code for custom roots in the values.yaml default from Helm from this chart.

Applicable issues

N/A

Additional information

N/A

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md